The present invention relates generally to determining the existence of one of a number of conditions on a bidirectional pin. More particularly, the present invention relates to a circuit used in graphics adapter cards to determine which of four types of digital to analog converters are connected thereto by reading logical values at a single pin.
It is known to decode one of two states at a single pin by determining whether the pin level is a logical one or zero. This technique has been fine for graphics adapter cards when only one of two types of digital to analog decoders (DACs) could be used. Prior art devices could use only an 8 bit DAC, only a 6 bit DAC, or provision could be made for either one of these DACs to present. This means that both conditions could be decoded by use of a single pin.
The recent proliferation of computing systems, with their diversity in hardware, have required that hardware manufacturers make provision for adaption of hardware with variable parameters on "as transparent as possible" basis. In other words, a graphics adapter card installed in a system, should be able to detect various attributes of the hardware it is to interface with and configure and operate appropriately. For example, programmable DACs have been developed which operate as an 8 bit DAC or a 6 bit DAC, but have the capability of operating in the alternate mode if selected. Typically, a default logic state is established by external circuitry to operate the DAC in one of the conditions.
As a result, it may become necessary to distinguish among more than two types of hardware. More pins is a solution, but pins are a scarce resource. A user selectable switch having multiple contacts to provide more than two binary conditions represents a solution, but is not transparent to a user who must determine the type of hardware and enter it correctly into the switchbank. Additionally, the user selectable switch approach requires more complex (and expensive) components, and requires additional digital logic to process and store the multiple bits of information from the switches.